In general, encoders at the drive shaft, on the one hand, and additional vanes and sensors in the hoistway, on the other hand, are used to detect absolute car position. In an emergency affecting the power source, the absolute position information can be written into an EEPROM or battery backed-up RAM to avoid the loss of that information. However, if the car moves independently of the elevator drive after the power supply has failed, or after getting the last absolute position signal, actual car position is lost. In such a case, after connecting to the line voltage, the absolute car position is usually obtained by means of an initialization run. U.S. Pat. No. 4,341,287 shows such a system. In other applications, multi-channel encoders are coupled to the car by a steel tape, holed or having magnets placed thereon, and the pulse train signals from the encoder are transformed to absolute position information. The absolute position initialization is accomplished by moving the car a few centimeters. Other prior art arrangements use coded indicia in the hoistway and appropriate readers of the indicia on the car or batteries for powering the absolute car position memory circuits during a power outage.
It would be desirable to determine the absolute car position, without requiring the car to move to a predetermined initialization floor, without requiring coded indicia in the hoistway and code readers on the car, and without requiring batteries or other auxiliary power supplies for storing the absolute position the car had before power failure.
There is a wide range of requirements for absolute position indicators, but not every requirement must be fulfilled. For instance, in cases of an emergency, it is enough to know the approximate location of the car. In order to get the absolute car position directly, the absolute position sensor should be located in the hoistway. This requires a sensor system, which is insensitive to dust and acoustical interferences. For this reason, optical methods, such as infrared and laser, are unacceptable. Optical sensors are sensitive to dust because the light intensity decreases where a layer of dust appears on the lens or reflector. In addition, there is a need of regular maintenance which increases costs.
An absolute position measuring system with ultrasonic sensors offering the advantage of being usable in dusty environments such as hoistways is described in co-pending application Ser. No. 07/709,796, a continuation-in-part of Ser. No. 07/695,364, "Static Measuring of Elevator Car Position", by C. Schmidt-Milkau, K. Disterer, and R. E. Hanitsch, and assigned to Otis Elevator Company.
A problem with acoustic methods is echoes. As all echoes have the same frequency and intensity as the direct signal, differentiating between direct signals and echoes is difficult. Where two transducers are used, a first transducer and a responding transducer, there are usually two types of echoes: near-echoes and far-echoes. Near-echoes distort measurements at the first transducer; far-echoes distort measurements at the responding transducer. Near-echoes can produce a signal which would indicate the end of the measurement. They do this by hitting some object and rebounding onto to the first transducer before the responding transducer responds with a signal. The length of the path followed by the near-echo may vary, especially with the cross-sectional area of the hoistway.
Far-echoes are acoustic signals emitted from the first transducer and, rather than proceeding directly to the responding transducer, hit the walls and then the responding transducer. Because these signals do not travel the shortest distance between the two transducers, measuring them can only distort the distance measurement. The length of the path followed by the far-echo may vary, especially with the crosssectional area of the hoistway and the vertical distance between the transmitting transducer and the responding transducer.
One method of dealing with this problem is disclosed in "Measuring Elevator Car Position Using Ultrasound" assigned to the same assignee as the present invention Ser. No. 07/709,796, a continuation-in-part of Ser. No. 07/695,364, by C. Schmidt-Milkau, K. Disterer, and R. E. Hanitsch. Two ultrasonic transducers are provided for measuring absolute position, one upon the ceiling of an elevator hoistway, and the other on top of an elevator car. In addition, two delay elements are provided. A start signal initiates the absolute position measurement and causes a first ultrasonic signal to be transmitted from the ceiling transducer to the car transducer. After receipt of the first ultrasonic signal by the car transducer and a far-echo delay for avoiding ultrasonic echoes from the hoistway walls and hoist ropes, a second ultrasonic signal of the same amplitude and frequency is transmitted from the car transducer to the ceiling transducer. The ceiling transducer receives the second ultrasonic signal and provides a stop signal. A delay element prevents the stop signal from reaching a timer until the end of a selectable time period. The timer, responsive to start and stop signals measures the travel time of the ultrasonic signals. The multiplication of the travel time of the signals with their velocity, and use of the two echo-avoiding delays, yields the car's absolute position while at the same time avoiding echoes.
A disadvantage of all ultrasonic measuring methods is the limited working distance due to the high damping in air of sound or ultrasonic waves. Their use is restricted to low-rise elevators. The damping in air is equivalent to [1.17.times.10.sup.-4 (frequency/kHz).sup.2 ] dB/meter at 25.degree. Celsius.
A disadvantage of the ultrasonic measuring system above is the need for an echo-avoiding system.
It is desirable to measure absolute elevator position with sound at distances of more than 100 meters. It is further desirable to accomplish this without the need for an echo-avoiding system.